Place the ground sirloin in a large mixing bowl and punch a well into the center of the meat. Fill the well with the egg, bread crumbs, onion, half of the chopped garlic, half of the parsley, the Parmigiano and Romano cheese, and a little salt and pepper. Mix the meatball ingredients until well combined yet not overmixed. Divide into 4 equal parts, then roll each part into 5 balls. Heat a large non-stick skillet over medium-high heat with the EVOO. Add the meatballs and brown on all sides, about 5 minutes.
Scoot the meatballs to the side of the skillet and add the remaining garlic and the red pepper flakes. Cook for 1 minute. Add the tomatoes, mix to combine. Shake the skillet to coat the meatballs in the sauce and simmer for 5 to 6 minutes more. While the meatballs are simmering, start the spinach.
Heat the medium-size skillet over medium-high heat with the EVOO. Add the onion, salt, and pepper. Cook for 1 minute. While the onion is cooking, transfer the defrosted spinach to a clean kitchen towel and wring all the liquid out. Add the spinach, chicken stock, and cream to the skillet. Season with nutmeg and cook for 2 minutes or until the mixture is thick.
Spread the spinach mixture over the bottom of the baking dish. Top with the meatballs and tomato sauce. Top that with the shredded provolone or Italian four-cheese blend. Place under the preheated broiler about 6 inches from the heat to melt and evenly brown the cheese about 3 minutes. Remove no-sagna from the broiler and garnish with lots of torn or shredded basil.